The cause of fatigue during prolonged exercise in the heat has not been clearly established, but performance appears to be limited by thermoregulatory and fluid balance factors. Both dehydration and hyperthermia separately impair performance. Prior acclimation reduces the impact of heat, and ingestion of fluids also improves performance. Ingested fluids should contain carbohydrate and some electrolytes, primarily sodium, but many factors will dictate the choice of fluids. These include the relative need for carbohydrate to supplement endogenous stores, the need for fluid replacement, the limitations on replacement imposed by the nature of the event, and the athlete's own requirements and preferences.